Beside the meaning of inventing or making up a new phrase, it is also used before you use a phrase which sounds slightly silly as in : " He was, to coin a phrase, as sick as a parrot."

To coin a phrase ( or a term) is to invent a figure of speech. Finley Peter Dunne was the first person to call a left-handed pitcher a "southpaw," and so he is said to have coined the term. Read more: http://wiki.answers.com/Q/What_is_the_definition_of_to_coin_a_phrase#ixzz1LqYtmKQc

The original meaning of 'to coin a phrase' was to invent a new phrase or expression. But now it is something you say before or after using an expression or quotation that has been very popular or used too much. ' I am, to coin a phrase, gobsmacked!' 'I have to say we are not amused, to coin a phrase.' (We are not amused is a quotation attributed to Queen Victoria)